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Do electrons push each other in a circuit ?

  1. Aug 27, 2013 #1
    i have a made a thread a few hours ago with something related to this
    but i would like to know
    do electrons push each other in a circuit ?
    that would explain the constant intensity across a circuit before electrons even reach a resistor .
    i mean they push each other like some kind of balls in a tube where they keep pushing each other to move and flow , where slowing one ball in the end of the line would end up causing the whole row to slow down
     
  2. jcsd
  3. Aug 27, 2013 #2
    It's approximately like that. In fact not only electrons but charged particles do exactly that because they have charge which interacts with other charge.
    it's like a line of magnets , they all have their fields around them now pushing one would push the other and so on.
    the only difference is that for balls o push on balls or other materials it doesn't happen that fast and changes in pressure are different but for electrons their charge interact via the electric field , and the electric field travels at the speed of c in vacuum and something very close to it in other materials, so a change in pressure at one end would be almoust felt instantaneously at the other end.
     
  4. Aug 27, 2013 #3
    one of the most helpful comments i have read today , thanks !
     
  5. Aug 27, 2013 #4

    sophiecentaur

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    The links on a bicycle chain are also an analogy. The tension after the pedals happens to be greater than the compression after the back wheel, of course, but dont forget, there is also 'tension' pulling the electrons towards the positive terminal of the battery because of the +protons in the metal.
    In any of these very crude models, it's essential to realise that the speed of the motion of the 'carriers' is very much slower than the speed at which the energy is transferred. Once you have accepted the fact that the coulomb forces between the charges are enormous and 'stretching or compression' of the column of electrons that people like to visualise is very small, then leave that all behind and get on with conventions of electrical circuit theory - because they work! When you're good and ready, move into QM and more details about conduction but don't expect it to help you with working out resistor networks :wink:.
     
  6. Aug 27, 2013 #5

    ZapperZ

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    Even though there are electron-electron interactions, this effect is quite small for most of the physical properties that we encounter in a metal. For example, the electron-electron interaction produces negligible effects in the resistivity of a metal at room temperature. In fact, within the Fermi Liquid theory, this many-body effects are normalized out via the electron's effective mass, resulting in a "transformation" from a single many-body problem to a many one-body problem.

    So, do electrons push each other in a circuit? In principle, yes. However, in practice, we often regard that to be too small that we neglect it in the majority of the cases.

    Zz.
     
  7. Aug 27, 2013 #6

    sophiecentaur

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    Is there not an equal amount of pulling done by the positive ion cores? It would seem that it's only a slight (only a finite amount is needed) imbalance in the spacing between the electrons that produces net repulsion and attraction forces to cause the drift.
    Or can it be argued that individual electrons can find themselves closer together than the lattice spacing so the repulsion effect can be greater?
    It always bugs me that so much time is spend on reiterating semi intuitive models, based on analogies. Marbles in a tube are OK for the first few minutes of an argument and may help to get somewhere but we should start with a huge caveat whenever doing it this way. As soon as such an analogy starts to 'creak a bit' it should be discarded - then move on.
     
  8. Aug 27, 2013 #7

    ZapperZ

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    I'm not sure what analogy you are referring to.

    The positive ion cores, in the Bloch model, are the periodic potential that gives us the Bloch wave function having the specific translational symmetry. This has more of an effect that causes the breakdown of the Drude model than the electron-electron interaction in an ordinary metal.

    The physics of many-electron phenomenon is not trivial. Strongly-correlated electron system is what condensed matter physics is all about. These are only apparent in more complicated, rich, exotic material, not in your ordinary conductors.

    Zz.
     
  9. Aug 27, 2013 #8

    sophiecentaur

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    Not any of yours!, don't worry :wink: I was getting at the over simple models which involve hand waving and marbles.
     
  10. Aug 27, 2013 #9
    i just wanted to know the system to get to know why the intensity is the same in a circuit with series resistors , just to have an idea of what's happening , whether the electrons push each other , or the magnetic fields interact , it would still hold the same idea , that electrons are not just flowing , they interact with each other
     
  11. Aug 27, 2013 #10

    ZapperZ

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    But it would be misleading to think that this is due to such interaction, as I've stated.

    Why not just consider something like conservation of charge? Everything that comes in, must come out to prevent charge accumulation?

    The idea of electrons interacting with one another is not as simple as you think. In fact it will add even MORE complications.

    Zz.
     
  12. Aug 27, 2013 #11
    If it makes things more understandable , charged particles do interact with each other and the field (electric field) is the medium thanks to which they can do that.
    Now what Zapper and Sophie started the debate here is that it's not the electrons themselves in the wire which make the net movement in all the circuit so fast and so strong it's the electric field acting on those electrons.
    The electric field is that "water" pressure from the pipe analogy if it makes things clearer.
    The stronger the field the more pressure it puts on the electrons.The field strength is basically in other words the potential or in every day language volts.
    The actual electrons move kinda slowly because they are not free in the metal and even those who are and form current do face resistance all the way so it's the field which propagates so fast when you apply a current at one end , so you get an almoust instantaneous effect at the other side of the circuit.

    the intensity is the same with series circuits has to do with current , the electric field may get weaker but amperage is amperage, it is the measure of how many electrons pass a given point in a given amount of time (usually 1 sec.) Now if your current started out with say x electrons and there is only one way or one path for them to go then they must go through that path until a potential equilibrium with ground is reached , but if there is a potential difference then they will still flow and so heat up each of the next series resistors.

    Like in cars the voltage may very well be low so the e field is also low but the amperage is high so things tend to get hot very fast , like resistors or shorted wires etc.
     
    Last edited: Aug 27, 2013
  13. Aug 27, 2013 #12

    sophiecentaur

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    The speed the electrons move (drift) is not to do with resistance. It's to do with carrier density and current (that formula, remember).Don't confuse the poor guy.
     
  14. Aug 27, 2013 #13

    sophiecentaur

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    Avoid the term "pressure". It's Potential that we're dealing with. Start with pressure and you'll end up with Kinetic Energy and be even deeper in the mire. Saying that Field is the same as potential is also very misleading. If you are trying to help people, at least try to talk the proper talk. The distinction between terms is relevant and is not just nitpicking. These guys can do without being mislead when they come to PF for help.
    There are perfectly correct explanations for all this stuff.
     
  15. Aug 27, 2013 #14
    dont worry i know what an electric potential is
    btw is the field caused by repulsion/attraction between charged particles ? if so what increases the strength of the field , the amount of charge or the amount of potential ? or is it something else ?
    he was trying to give me an idea which is pretty helpful , sometimes if you cant to make someone understand the perfect truth , you add some icing of lies on realities * faulty analogies for example , like the water pressure analogy * to get a somehow close basic idea
     
  16. Aug 27, 2013 #15

    sophiecentaur

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    What's wrong with the correct version?
     
  17. Aug 27, 2013 #16
    i didnt really mean interactions , like quantum mechanical interactions or anything , i meant simple electric repulsion , that prevents electrons from piling up , if there was no interaction or whatsoever , they would flow with high intensity in the piece of wire that has not reached the resistor * since the intensity is equal to voltage divided by resistance and the resistance is near zero * , but for an electron in the beginning of the circuit to 'feel' the resistance , there must be something to somehow * transmit * resistance . right ? other wise , if there was no interactions , then what tells electrons what intensity to flow with so they wont pile up ? there must be something that tells the CHARGE that there is resistance across the wire , i think this something is the electric repulsion force pushing charges that has not reach the resistors back and somehow slowing them or decreasing their intensity , if this is also not the case , then i guess i made a very huge mistake thinking of intensity in terms of speed !
     
  18. Aug 27, 2013 #17
    nothing its just i think i am not understanding the correct one , so he's trying to get me an analogy to make things easier
     
  19. Aug 27, 2013 #18
    Speed is not a good way to think about it. You should rather think in terms of current. A small number of electrons moving very fast and a large number of electrons moving slow can constitute the same current.

    It's all about fields. Charge piles up at the resistor causing a field that opposes the one from the battery. It's correct that charge carriers (electrons in this case) can interact through their fields but you shouldn't think of this as a microscopic effect i.e. one electron pushing the one next to it. It's a macroscopic effect. A large number of electrons cause a charge imbalance and therefore a field that extends over some distance (the entire circuit).

    Instead of metal wires imagine a circuit using plactic wires. Plastic has a very high resistivity. So in that case everything goes very slowly. It takes a while for the current to become equal in the whole circuit. When you connect the battery, at first the field is strong close to the terminals of the battery and weak further away. So you have different field strengths in different parts of the wire. That means the current caused by the field will be different. But if you wait long enough eventually an equilibrium will be reached where the current is equal everywhere in the wire. That happens because charge piles up in the wire in such a way that the field becomes equal everywhere in the wire.
     
  20. Aug 27, 2013 #19
    so its all about creating an equillibrium ? to evade creating a pile up of electrons ?
    i think it sounds for me something like this , please correct me if i am wrong .
    there is a row of cars that has to pass an unpaved portion of a road , say for instance that the whole road is paved * with resistance almost close to zero* and then they have to pass through that unpaved part * resistor* , so the first car slows down due to it's passing through the tough road , so it moves with a speed 20 m/s * i know i shouldn't be thinking of it as speed , but just to make things clearer*
    now if we look to the cars behind it , they are also moving at 20 m/s even though they are moving on a paved road , but they do this not to pile up
    how correct is this analogy ?
     
  21. Aug 28, 2013 #20

    ehild

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    That is a good analogy to explain while the current is the same everywhere in series connected resistors and connecting wires. In the conductor, the electric interaction among the electrons and ions and between the electrons tends to keep neutrality. The charge density in any volume element becomes zero after a short transient period when stationary current flows.

    Your cars notice when they have to slow down from the rear lights of the one in front of them. If the first car breaks, the next notices it, and also breaks, and the next sees it also, and so on. The "break" signal travels very fast, so the whole queue of cars moves practically together, as the current of electrons follows the varying electric field .

    ehild
     
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